I want to add that Microsoft documentation appears to be incorrect regarding this as well. This article shows you how to install a subordination CA and save an offline request in the GUI. It then shows the PS commands to do the same thing, but the PS command does not include the OutputCertRequestFile parameter and it does not explain if and where a req file is saved when using the command.

"The Windows PowerShell commands to perform the installation of the Enterprise Subordinate CA as shown in this section are":